1. Field of the Invention
The present invention relates to a thermal printing method and thermal printer, and more particularly, relates to a thermal printing method and thermal printer capable of efficient transfer of data and in which reproducibility of gradation can be high also at a high printing speed without being lowered.
2. Description Related to the Prior Art
As known examples of thermal recording, there are thermal transfer recording and direct thermal recording. In the thermal transfer recording, a thermal head applies heat to ink film and transfers ink to recording material. In the direct thermal recording, a thermosensitive type of the recording material is used and heated by the thermal head to develop color. In both types of the thermal recording, the thermal head in the thermal printer includes a plurality of heating elements in an array extending in a main scan direction. One of the thermal head and the recording material is fed to the remaining one of them in a sub scan direction, while the heating elements are driven to record an image to the recording material one line after another. For the thermal recording, heating data is referred to for controlling heat to be generated by the heating elements. So density of dots on the recording material is varied to reproduce gradation of pixels with fidelity.
A full-color type of the thermal printer is for use with a full-color type of the recording material, which is constituted by a support, cyan, magenta and yellow thermosensitive coloring layers, and a protective layer, all overlaid in sequence. The thermosensitive coloring layers are different in heat energy required for developing color. Higher heat energy is required according to the depth of each position of the coloring layers in the thermal printer. The coloring layers are heated selectively. Before the second and third of the coloring layers are heated, respectively the first and second of the coloring layers are subjected to application of ultraviolet fixation rays, and are prevented from further developing the color. The three colors are recorded to the coloring layers so as to print a full-color image to the recording material.
To record each one dot to the coloring layers, the heating elements apply bias heat energy to the recording material, the bias heat energy being enough for heating the recording material to a state directly short of starting color development. After the bias heating, the heating elements apply gradation heat energy to the recording material, the gradation heat energy being determined by density at which each color should be developed. This combination of the bias heating and gradation heating records a dot by coloring each one of pixels, which are virtually defined on a surface of the recording material as quadrilateral cells arranged in a matrix form.
The thermal printer includes a frame memory, to which image data from a digital still camera, personal computer or the like is written. At the time of writing the image data, one-line image data is read from the frame memory and written to a line memory. Then a comparator compares heating data with gradation level data which is stepped up one by one. The comparator outputs the heating data of a serial signal as a result of the comparison, which is transferred to the thermal head.
The thermal head includes the heating element array and a driver, which controls heat energy for each of the heating elements according to the heating data. The driver converts the heating data of the serial signal into a parallel signal, and turns on and off the heating elements.
In the thermal printer mentioned above, the image data in 256 gradation levels is converted into the serial signal and transferred to the thermal head. To this end, comparison is effected for 256 times between the image data and the gradation level data, to transfer result of the comparison serially. The number of times of the transferring process of the heating data for the one line is 256. In other words, the number of times of the transferring process is equal to the number of the gradation levels for the maximum density in of the image data. If it is intended to raise reproducibility of the gradation, for example from the 256 gradation levels to 512, then the data transferring time becomes longer, and becomes twice as long as that according to the 256 gradation levels. There occurs a problem in that the printing speed is decreased.
In view of the foregoing problems, an object of the present invention is to provide a thermal printing method and thermal printer in which reproducibility of gradation can be high also at a high printing speed without being lowered.
In order to achieve the above and other objects and advantages of this invention, a heating element array has plural heating elements. A head driver drives the plural heating elements according to heating data for respectively the plural heating elements, to record dots of one line thermally by heating thermosensitive recording material. For thermal printing, a line memory stores one-line image data for plural pixels in the one line. An even number gradation counter sequentially outputs gradation level data of gradation level N and gradation level data of gradation levels changed serially by two from the gradation level N, where N is an integer. A first comparator serially compares the one-line image data with the gradation level data from the even number gradation counter, so as to create even number gradation heating data in a serial signal form. An odd number gradation counter is operated in synchronism with the even number gradation counter, for sequentially outputting gradation level data of gradation level N+1 and gradation level data of gradation levels changed serially by two from the gradation level N+1. A second comparator serially compares the one-line image data with the gradation level data from the odd number gradation counter, so as to create odd number gradation heating data in a serial signal form. The head driver includes a first converter for converting the even number gradation heating data into a parallel signal form. A second converter converts the odd number gradation heating data into a parallel signal form. A drive signal generator supplies the heating elements with respectively a drive signal by alternately reading the even and odd number gradation heating data from the first and second converters.
According to a preferred embodiment, the first and second converters comprise first and second shift registers.
Furthermore, a strobe signal generator generates a strobe signal at a regular period. Also, N=0. When P strobe signals are generated after each of the gradation level data is output, the even and odd number gradation counters output a succeeding one of the gradation level data at a gradation level increased serially, where P is an integer equal to or more than one.
The drive signal generator includes an even number counter for generating an even count signal if the strobe signal number is even. A first latch array is connected with the first shift register, for latching the even number gradation heating data in response to the even count signal. An odd number counter generates an odd count signal if the strobe signal number is odd. A second latch array is connected with the second shift register, for latching the odd number gradation heating data in response to the odd count signal. An OR gate array obtains integer gradation heating data in a parallel signal form by OR operation of the even and odd number gradation heating data from the first and second latch arrays, to determine the drive signal according thereto.
According to another preferred embodiment, a line memory stores one-line image data for plural pixels in the one line. An even number gradation counter sequentially outputs gradation level data of gradation level N and gradation level data of gradation levels changed serially by two from the gradation level N, where N is an integer. A first comparator serially compares the one-line image data with the gradation level data from the even number gradation counter, so as to create even number gradation heating data in a serial signal form. An odd number gradation counter is operated in synchronism with the even number gradation counter, for sequentially outputting gradation level data of gradation level N+1 and gradation level data of gradation levels changed serially by two from the gradation level N+1. A second comparator serially compares the one-line image data with the gradation level data from the odd number gradation counter, so as to create odd number gradation heating data in a serial signal form. A combined heating data generator creates combined heating data in a serial signal form according to the even and odd gradation heating data, the combined heating data being any one of first, second and third information different from one another. The head driver includes a decoder for converting the combined heating data into even and odd gradation heating data. A first converter converts the even number gradation heating data into a parallel signal form. A second converter converts the odd number gradation heating data into a parallel signal form. A drive signal generator supplies the heating elements with respectively a drive signal by alternately reading the even and odd number gradation heating data from the first and second converters.
The combined heating data generator includes a first latch circuit for latching the even number gradation heating data in the serial signal form to output the first or second information in a binary manner. A second latch circuit latches the odd number gradation heating data in the serial signal form to output the first or second information in a binary manner. An information generator circuit is operated if outputs from the first and second latch circuits are equal to one another, for outputting the first or second information in a through output manner according to the outputs of the first and second latch circuits, and operated if the outputs from the first and second latch circuits are different from one another, for outputting the third information, the first, second and third information constituting the combined heating data.
The combination of the even and odd gradation heating data is any one of 00, 01 and 11, and the combined heating data is the first information if the combination is 00, is the second information if the combination is 01, and is the third information if the combination is 11.
The first information is 0, the second information is xe2x88x921, and the third information is 1.